The Hubble WFC3 Emission Spectrum of the Extremely Hot Jupiter KELT-9b

Abstract Recent studies of ultra-hot Jupiters suggested that their atmospheres could have thermal inversions due to the presence optical absorbers such as titanium oxide (TiO), vanadium (VO), iron hydride (FeH), and other metal hydride/oxides. However, it is expected these molecules would thermally dissociate at extremely high temperatures, thus leading featureless spectra in infrared. KELT-9 b, hottest exoplanet discovered so far, thought belong this regime host an atmosphere dominated by neutral hydrogen from dissociation atomic/ionic species. Here, we analyzed eclipse spectrum obtained using Hubble Space Telescope’s Wide Field Camera 3 and, utilizing atmospheric retrieval code TauREx3, found consistent with molecular species poorly fitted a simple blackbody. In particular, find combination TiO, VO, FeH, H - provides best fit when considering Telescope (HST), Spitzer, TESS data sets together. Aware potential biases combining instruments, also HST alone TiO VO only were needed case. These findings paint more complex picture planets than previously thought.